Alkyl polyglycosides (alkylpolyglycoside) are nonionic surfactants prepared by glycosidation of a source of a carbohydrate with an alcohol. alkylpolyglycoside contain one or more monosaccharide units and an alkyl side chain. Alkyl polyglycosides have the general structure: ##STR1## The term "alkylpolyglycoside", as used herein, includes alkyl monoglycosides, alkyl oligoglycosides (2-10 sugar units), and alkyl polyglycosides. The average alkyl chain length results from the alcohol feedstock, the particular synthetic method employed, and the extent to which the reaction is driven to completion. The degree of saccharide polymerization (DP) is due to the fact that monosaccharides also contain a primary hydroxyl at the C.sub.6 position. alkylpolyglycoside having a DP less than 2 are preferred for good surfactant surface tension.
Alkylpolyglycosides are typically prepared by reacting monosaccharide such as glucose or a monosaccharide source such as dehydrated starch syrup with a long chain primary alcohol in the presence of a strong acid catalyst. The known two-stage process involves the following basic steps: 1) acid catalyzed glycosidation reaction of a monosaccharide source with butanol to form butyl glycoside, with removal of water formed during the reaction, 2) transglycosidation of the butyl glycoside with a C.sub.8 to C.sub.20 alcohol to form the long (alkyl) chain alkylpolyglycoside, with removal of the butanol, 3) neutralization of the acid catalyst, 4) distillation to remove unreacted long chain alcohol, 5) bleaching to improve the color and odor of the product and 6) isolation of the alkylpolyglycoside. The glycosidation and transglycosidation reactions are equilibrium controlled until the catalyst is neutralized.
The typical single-stage alkylpolyglycoside process involves all of the steps of the two-stage process, with the exception of the consolidation of steps 1) and 2) shown above by directly reacting glucose with a long chain alcohol.
The commonly preferred known catalysts for the two-stage process are sulfuric acid and p-toluene-sulfonic acid, as disclosed in U.S. Pat. Nos. 3,772,269, and 3,375,243. It is a generally known that strongly acidic reaction conditions are required for successful glycosidation. Favored catalysts for single-stage processes have been long chain alkyl sulfonic acids since they have improved solubility with both the carbohydrate and alcohol. For example, see U.S. Pat. No. 5,459,249. It is known that choice of catalyst is critical both to the color and the DP of the final alkylpolyglycoside product.
Alkylpolyglycoside glycosidation reactions catalyzed by strong acids such as sulfuric acid suffer from two main sources of color formation. Firstly, the strong acid catalyst promotes dehydration of the monosaccharide. For instance, when the monosaccharide is glucose, dehydration leads to highly colored furan derivatives such as furfuraldehyde. Secondly, the strongly acidic (pH of about 1.0) acid catalyzed reactions must be neutralized with a strong base in order to terminate the reaction. While the addition of base is necessary to stop the glycosidation equilibrium reaction, the neutralization reaction promotes unavoidable formation of color via the well known "peeling reactions".
U.S. Pat. No. 5,432,269 discloses binary catalysts formed from a strong organic acid and a weak organic base that provide improved alkylpolyglycoside color. However, the reaction medium is still strongly acidic throughout the reaction period. The reaction medium must be neutralized at the end of the reaction. In addition to raising the pH of the reaction medium, neutralization also liberates the organic base which must be removed by distillation. Unfortunately, neutralization also leads to color formation.
U.S. Pat. No. 4,898,934 discloses the addition of alkali salts of inorganic acids or polybasic saturated carboxylic acids to the reaction mixture containing an acid such as sulfuric acid. The additives are reported to act as complexing agents. The acid must still be neutralized at the completion of the reaction. The addition of base promotes unavoidable side reactions, namely, "peeling reactions" which contribute to excess color formation.
U.S. Pat. No. 4,465,828 discloses the addition of hydroxy-carboxylic acids to the reaction mixture as a means of decreasing color formation in the production of alkylpolyglycoside. The carboxylic acid reportedly serves as an acid buffer.
U.S. Pat. No. 4,704,453 discloses the complete neutralization of an acid such as H.sub.2 SO.sub.4 with an equivalent or excess amount of sodium metaborate, borax, or sodium perborate. Boric acid is thereby generated from sulfuric acid. The boric acid is disclosed to complex with glucose, forming a Lewis acid catalyst, which is the actual reaction catalyst. However, the borate salts formed by that reaction must be removed from the reaction medium.
The commonly preferred means of alkylpolyglycoside color reduction are bleaching with 30% aqueous peroxide, as disclosed in WO 9003077, WO 9402494, U.S. Pat. No. 5,432,275, and U.S. Pat. No. 5,362,861; bleaching with an aqueous solution of ozone, as disclosed in EP 569682, DE 3940827, and EP 389753; and reduction with boranates, as shown in U.S. Pat. No. 5,104,981, and U.S. Pat. No. 5,430,131.
In alkylpolyglycoside glycosidation, the isolation step has also been the subject of much research. The vast majority of the time, the product is isolated and packaged as an aqueous solution. However, a number of research groups have attempted to find conditions for isolating the alkylpolyglycoside as a solid or in a highly purified form by extraction because of the increased cost of transporting aqueous solutions and the formulation constraints which are associated with isolation of products as aqueous solutions. U.S. Pat. No. 3,547,828 discloses an acetone extraction method in which a crude alkylpolyglycoside mixture is fractionated into a solid high DP fraction (DP equal to or greater than 2), which is isolated by filtration or centrifuging, and a viscous liquid composed primarily of alkylmonoglycoside (a low DP fraction having a DP less than 2) plus excess alcohol and acetone. In order to isolate the low DP alkylpolyglycoside from the acetone fraction, excess free alcohol and acetone are distilled away. Distillation, of course, leads to discoloration of the low DP alkylpolyglycoside. Distillation can be avoided by removing the acetone by simple distillation and recycling the alcohol/low DP alkylpolyglycoside back to the reaction vessel. Recycling lowers the overall yield of the reaction and the low DP alkylpolyglycoside is not available for use as a surfactant.
In light of the above, it would be desirable to have an easier, more efficient process for producing and isolating low color alkylpolyglycoside. It would be particularly desirable to provide such a process wherein the color and odor reduction step is completely avoided. It would be further desirable if such process included an efficient, high yield method of isolating the alkylpolyglycoside formed and separating the alkylpolyglycoside product into a solid or near solid, high DP alkylpolyglycoside component and a solid or near solid low DP alkylpolyglycoside component.